Question: Simplify the following expression and state the conditions under which the simplification is valid. You can assume that $a \neq 0$. $n = \dfrac{-3a + 24}{a + 6} \times \dfrac{2a^2 + 2a}{a^2 - 7a - 8} $
Answer: First factor the quadratic. $n = \dfrac{-3a + 24}{a + 6} \times \dfrac{2a^2 + 2a}{(a - 8)(a + 1)} $ Then factor out any other terms. $n = \dfrac{-3(a - 8)}{a + 6} \times \dfrac{2a(a + 1)}{(a - 8)(a + 1)} $ Then multiply the two numerators and multiply the two denominators. $n = \dfrac{ -3(a - 8) \times 2a(a + 1) } { (a + 6) \times (a - 8)(a + 1) } $ $n = \dfrac{ -6a(a - 8)(a + 1)}{ (a + 6)(a - 8)(a + 1)} $ Notice that $(a + 1)$ and $(a - 8)$ appear in both the numerator and denominator so we can cancel them. $n = \dfrac{ -6a\cancel{(a - 8)}(a + 1)}{ (a + 6)\cancel{(a - 8)}(a + 1)} $ We are dividing by $a - 8$ , so $a - 8 \neq 0$ Therefore, $a \neq 8$ $n = \dfrac{ -6a\cancel{(a - 8)}\cancel{(a + 1)}}{ (a + 6)\cancel{(a - 8)}\cancel{(a + 1)}} $ We are dividing by $a + 1$ , so $a + 1 \neq 0$ Therefore, $a \neq -1$ $n = \dfrac{-6a}{a + 6} ; \space a \neq 8 ; \space a \neq -1 $